1. Field of the Invention
The present invention relates to fluorescent displays manufactured using a film with a low plasticizer content made of polyvinyl acetal and a plasticizer-containing film made of polyvinyl acetal.
2. Background Art
In order to mirror information into the driver's visual field, so-called “heads-up displays” (HUDs) are currently used in passenger vehicles. For this purpose, an image is projected from a projector unit located in the dashboard support to the windshield (WS) that can be seen as a virtual image from the driver's seat. The effect is based on the reflection effect of the glass surface oriented toward the passenger compartment and the glass surface oriented toward the vehicle exterior. The inner glass surfaces contribute almost nothing to the reflection, since they, with their legally prescribed compound safety glass, are adhered to a PVB film located between the layers of glass which has a similar refractive index similar to glass.
In order to simplify references to the four glass surfaces of a conventionally structured windshield, the following convention is used which is shown in FIG. 1:
Side 1: outer surface of the pane of the composite body oriented toward the vehicle exterior;
Side 2: inner surface of the outer pane in the composite body oriented toward the intermediate layer film;
Side 3: inner surface of the inner pane in the composite body oriented toward the intermediate layer film;
Side 4: surface of the inner pane of the composite body oriented toward the vehicle interior.
In general, a film made of a plasticized polyvinyl butyral, abbreviated as “PVB film,” is used as the intermediate layer film.
In order to eliminate double images in conventional HUDs with the described functional principle, the glass surfaces associated with sides 1 and 4 must be at a certain angle with respect to each other. This is achieved through the use of PVB films with a wedge-shaped thickness profile. However, such films are difficult to work with and tedious to manufacture, which makes them substantially more expensive than non-wedge-shaped film counterparts.
Other drawbacks of conventional HUDs include the fact that the information can only be viewed in a limited field of vision directly in front of the driver and is also only visible to the driver. There are many situations, however, in which information is to also be visible to passengers or displayed at another location of the windshield. For example, it is conceivable to project safety-relevant information directly onto the windshield, such as the contour of obstacles located in or next to driving surface such as those already detected in current automobiles by means of night vision systems. In a conventional HUD, the image is distorted by raindrops on the windshield, because the refractive index transition between glass and air underlying the optimized system configuration is distorted.
As an alternative to these conventional HUDs, it has already been proposed to generate images by means of fluorescing materials arranged in the windshield and the targeted excitation thereof (so-called “fluorescent HUD”). The excitation of these materials is done by means of UV radiation invisible to the human eye, whereby real images visible not only to the driver can be generated on the plane of the windshield. Appropriate organic dyes, inorganic particles, etc., have been proposed as fluorescing materials.
For the manufacture of fluorescent HUDs, it is known to apply fluorescing pigments or dyes in the form of coatings, laminated films or the like to side 4 of the windshield. Alternatively, it has been proposed to position fluorophore-containing films between side 3 and the PVB intermediate layer or to print the surface of the PVB film oriented toward side 3 with fluorophores.
WO 2012/072950A1 discloses printing a plasticizer-containing PVB film with certain low-molecular fluorophores such that they are distributed evenly in the film during the autoclaving process. The fluorophores are intended to act simultaneously as UV absorbers. In this respect, WO 2012/072950A1 describes the use of low-molecular hydroxyterephthalate esters, particularly 2,5-dihydroxydiethylterephthalate, as fluorophores and antioxidants that are applied to a PVB film. The fluorophore is distributed in the PVB film and can either act as a UV absorber itself or be mixed with a low-molecular UV-absorber present in the film.
It is known from WO2008132368 A9 that fluorescent HUDs can contain one or more inorganic luminophore particle-containing layers in thicknesses of less than 20 μm on side 3 of a composite body in addition to a conventional plasticizer-containing PVB film. However, there is the possibility here of uncontrolled excitation occurring as a result from sunlight from the outside, since conventional plasticizer-containing PVB films usually have a residual transmission for UV radiation. Moreover, care must be taken that no unsuitably large particles are included that could lead to scattering of light. The small layer thickness of the luminophore-containing layers of 20 μm limits the quantity of fluorophores that can be used, since greater concentrations lead to cloudiness. Consequently, the maximum achievable intensity of the fluorescent emission is limited.
US 2002/0120916 A1 describes head-up displays with fluorescent dyes in which the side facing toward the observer contains a UV absorber. This is intended to prevent excitation radiation for the fluorescent dyes to radiate through the display to the outside. For this purpose, US 20110073773 A1 discloses fluorescent displays having an opaque material on the side of the display facing away from the observer. EP 2409833 describes a fluorescent display in which the fluorescent dyes on the side facing toward the sunlight.
Several of the proposed solutions according to the prior art have the additional drawback that it is necessary to work with several different films, not all of which can be adhered in a trouble-free manner to a glass surface. For example, while a sufficiently thin PET film can be printed with a fluorescent dye and such a dye can also possibly be mixed directly in the PET, PET is not inherently adhesive to glass surfaces, so there is the need to work with additional adhesion-promoting layers. Obviously, such a thin PET film would then have to be embedded between two PVB films. But this increases the costs, the propensity to defects and an increased effort in the manufacture of corresponding glass laminates. What is more, PET is substantially stiffer than conventional plasticizer-containing PVB films and therefore forms waves and folds in combination with curved glasses. The excessive stiffness of PET also remains in contact with plasticizer-containing films when undergoing a laminating process typical for the manufacture of composite safety glass, since PET is not capable of absorbing plasticizers, thus becoming more flexible.